In computed radiography (CR), an imaging plate having a storage phosphor, is exposed to an x-ray image of an object, such as an anatomical structure of a patient, to record a latent x-ray image in the storage phosphor. The latent x-ray image is read out by stimulating the storage phosphor with stimulating radiation of a first wavelength range by a laser. Upon stimulation, the storage phosphor emits radiation at a second wavelength range in proportion to the quantity of x-rays received. The storage phosphor is scanned in a raster pattern by a laser beam deflected by an oscillating or rotating mirror. The emitted radiation is reflected by a mirror light collector and detected by one or more photodetectors to produce an electronic image signal. Typically, the storage phosphor is translated in a slow or page scan direction past the laser beam which is repeatedly scanned in a fast or line scan direction perpendicular to the page scan motion of the storage phosphor.
As the spot size of the scanning laser beam of a CR reader is reduced, the depth of field of the system is reduced and accurate focus becomes important. This is particularly true as CR systems go to 50 um or smaller laser beam spot sizes. If the CR reader is provided with means to shift the laser optics to storage phosphor distance to adjust focus, a means of determining when the system is at best focus is needed. Doing a focus series and measuring MTF (Modulation Transfer Function) at each position can determine the best focus but is laborious. An easy and low cost means to check and optimize focus is needed during assembly and field repairs or upgrades. To minimize cost, it is desirable to utilize the light collector, photodetectors, and digital electronics of the CR reader for measuring the signal from the focus measuring assembly. Since radiation collected to the photodetectors is normally filtered at the photodetector(s), to remove the scanning radiation, a means must be provided to detect the focus measuring radiation.
U.S. Pat. No. 5,420,441, issued May 30, 1995, inventors Newman et al., is directed to an automated method for analyzing the photometric calibration and image quality performance characteristics of a high resolution storage phosphor reader. A custom fabricated, lead mask test target is secured to a storage phosphor and exposed using a standard medical x-ray source. A storage phosphor reader, to be calibrated, reads the exposed storage phosphor to produce a digital x-ray image. The digital x-ray image is analyzed by means of a computer algorithm which presents analytic results in text and graphical form to isolate reader problems and to quantify calibration status of the reader.
U.S. Pat. No. 5,591,968, issued Jan. 7, 1997, inventor Grillet, discloses an image plate coated with a photostimulable element and a method of testing a digital device for scanning such plate with respect to various system parameters. A permanent and non-erasable test pattern is applied to the plate for subsequent scanning by the device.
U.S. Pat. No. 4,459,002, issued Jul. 10, 1984, inventors Sakai et al., discloses a focus detecting system which has a signal processing circuit for processing output signals from photoelectric transducer elements. The signal processing circuit has a circuit for obtaining an absolute value of the output signal and for producing power thereof.
U.S. Pat. No. 3,912,396, issued Oct. 14, 1975, inventor Hartmann, discloses an apparatus for measuring the modulation transfer function of an image-forming lens or lens system. The apparatus uses an object target which is of the type having parallel light lines on a dark background. The object target is illuminated and the lens is mounted, e.g., an equal number of focal lengths from the target to focus an image at an image plane. An opaque plate member having an elongate narrow slit parallel to the light lines is mounted at the image plane for lateral movement of the slit at a uniform speed across the image plane. The differences in illumination at the slit, particularly as the slit travels from dark to light and light to dark areas of the image, is detected by a photoelectric converter which outputs an electrical signal defining the modulation properties of the lens. The signal can be amplified and read in an oscilloscope for visual examination or compared with standard signals in a computer.
While such systems may have achieved certain degrees of success in their particular applications, there is a need to for a focus determining system in a computed radiography reader or other laser scanning system, which is cost efficient, which utilizes existing system components where appropriate, and which is relatively simple to use.